Piezoelectric crystal holder



May 9, 1950 R. E. FRANKLIN x-:T AL

PIEZOELECTRIC CRYSTAL HOLDER Filed Nov. 26, 1947 1A/RJ Z 0 9 7 Y 2 2 oKuA E Nl N m WFL 1 2 A6 4 im 1MM A 2J n D Il L 0.* u w. Mmm/ Y y My/@ge.Mu @,wf 3 F Hm 2% \IIII\ 6 2 2 4 m n0 r l l l l l l l I I l l l l l l Il I IIL v. F ---f M Patented May 9, 1950 UNITED STATES 'QATENT OFFICEPIEZOELECTRIC CRYSTAL HOLDER Walle Application November 26, 1947, SerialNo. 788,256

7 Claims. (Cl. lll-327) This invention relates to a new and usefulpiezoelectric crystal holder for supporting crystals 4which willoscillate in the length-width mode.

The purpose of this invention is to improve the crystal frequencystability by (1) decreasing the loading due to crystal electrodes (whichare normally plated onto the crystal faces), (2) obtaining an unusuallyhigh Q factor, (3) improving the temperature frequency characteristicsof the holder, and (a) eliminating the electric twinning which is apt tooccur due to the local application of heat in the process of solderingwires to the silver plating on the crystal.

A feature of this invention is the improved arrangement of placing theelectrodes on auX- iliary plates of quartz or ceramic material which areadjacent to the crystal oscillator element instead of plated on thecrystal. It has been found that crystals which oscillate in alengthwidth inode may be clamped between rigid supports at the center ofthe crystal, provided the axis of clamping is normal to the plane ofmotion of the crystal. Such a clamping arrangement will load the crystalvery little, if the area of clamping is Very small in proportion to thearea of the crystal.

Piezoelectric crystal oscillators of the above type are sometimes usedas primary standards of frequency, or as the frequency control elementin crystal driven clocks. For eiiicient operation, these crystals shouldhave, as nearly as possible, zero temperature coenicient of frequencyover a Wide range in temperature, and should also have the highest Qpossible. A high Q for a crystal oscillator is obtained by severalmeans. (l) After it is ground to as near correct frequency as possible,it is etched in a solution of some iluorine salt to free it frompossible loose particles of quartz, loosened in the process ofgrindingthis etching also reduces the amount of frequency drift calledaging, and 2) keeping the loading of the crystal at a such as mountingthe holder in a vacuum, keeping the clamping area small, and eliminatingthe loading due to electrodes, as much as possible.

The invention can be more clearly understood by referring to theaccompanying drawings, wherein:

Fig. l is an elevation of the crystal holder of this invention;

Fig. 2 is a cross-section of Fig. l;

Fig. 3 is an elevation of a modified form of this invention employingrectangular rather than circular electrodes;

Fig. 4 ls a cross-section of Fig. 3;

Fig. 5 is an enlarged detail elevation showing the method of supportingthe crystal at the central portion thereof; and

Fig. 6 is a side elevation of Fig. 5.

Referring now in detail to Figs. l and 2 of the drawings, therectangular crystal element I is of a type which oscillates in alength-width mode (when the length increases, the width decreases foreach half cycle; and the width increases and the length decreases forthe other half cycle of an alternating current wave). Circular members 2are the electrode supports of insulating material, preferably of fusedquartz, each support having a depression 2A ground on its inner face forlocating the spacing members 4. The electrode supports 2 each have theelectrode metal 3 deposited on the inner flat surface by chemical orevaporation means, which process is well known in the art. The spacingmembers 4 provide an air gap 5 between the crystal and the electrodewhich is divided equally on each of the two sides of the crystal. Thewidth of the air gap is adjusted by changing the length of the spacingelement 4, ywhich is also of insulating material such as crystallinequartz cut of twinned quartz. To prevent any possibility of undesiredthickness oscillations occurring in the spacing crystal il, they are cutfrom the mother crystal at such an angle that they have the sametemperature coeiiicient of expansion as the crystal element I. Theundesired oscillations are thus prevented as any crystalline quartzwhich has twinning, either optical or electrical, will not oscillate ifthe twinned-regions are anywhere near equal in area, and in the case ofspacing members i (which are quite small) almost any amount of twinningwill prevent oscillation in such members under any conditions of tuning.If the spacing crystal member fi is of crystalline quartz cut in thisway, it will more nearly retain a constant gap width with changingtemperature. Usually, variation in gap width will inuence the frequencyof the oscillating member I by changing the capacity in the equivalentcircuit of the crystal. Constant air gap width is desired as it iseffective in maintaining a constant frequency.

The surface of the crystal spacing member 4 in contact with the crystalhas a small depression which holds a very small piece `of insulationmaterial, or an insulating circular bead The insulating material ofmember' may be of fused quartz or'ceramic. If the electrode supportmembers 2 are round (as shown in Fig. 1), the small insulation member 6may be spherical and arrangedrto t equally into the depressions inmember 4 and the crystal element I. If the crystal assembly is arrangedas shown by Figs. 1 and 2 of the drawings, suitable ciamping means areapplied to the outer surface of electrode'supportmembers 2; the: entireassembly may be enclosed within a vacuum chamber (not shown).

The modications of Figs. 3 and 4 show a1 holder similar to that of Fig.1 lbutfwithrectan- .gular support members l2 and metallized electrodesI3 instead of the circular supportmenrbers and electrodes. Thisarrangement will give a somewhat smaller electrical gap capacity, for

the reason that the crystal gap capacity is determined by the (l) areaof the. electrodes facing each other, (2) the distance between theelectrodes, and (3) the dielectric constant of" the material in the gap,as quartz, air, etc. In the case of the rectangular holder; the totalicapacity is composedA or three capacities in series, that of each airgap-andthe largercapacity of' the crystal itself; while in thecaseoi'fthe circular holder; there are these three capacities inseriesand the capacity of the two unoccupied regions of this holder inparallel with the overall' capacity of the gap.- Fig. 4 also shows amounting means and ameans for holding the completedy unit together;applied to support the structure shown in Figs. 1 and12, as mentionedabove.

Iheelectrodes are formed on the surface of support membersI I2 bysuitable chemical or evaporation means. A similar depression IZA isprovided on the support members I2V for retaining spacing members I4'therein. Because ofthe rectangular shape'of' support membersv I-Zitwilll be necessary to prevent rotation of the crystal upon the supportmember. Therefore, the spacing members It are provided withalongitudinally shapedfbead Itf which fits equally into the slotteddepressions in' members I 4 and the crystal I, as isshown-in Figs. 5 and6. Thex mounting means Il'isof insulating material, preferably U-shaped,

withv an adjustable metallicl thumb screw I8 for fixing the pressurenecessaryto hold the various parts together and' to-ma-intain properclamping pressure. Metal contact springs I`9 are secured' totheinsulating member Il by means ofk bolts Z- and` nuts 2li. The springcontacts I9= make contact with the electrodes` 22, and, ifv desired,they may be solderedto the metallized surface.

In such a type of crystal holderthe metal parts that' mightinfluencerthe capacity of the equiva-V lent oscillating circuit havebeen reduced to a minimum, and by removing-theY crystal electrodes fromthe oscillating member, the Q of the crystal willi be greatly improved.Thecomplet'e crystal holder shown in Figs. 3V and elvis approximatelythesizeof a 100rkilocycle crystal. The-crystal holder is mounted in arather small evacuated chamber 23g shown by the broken lines, afterthevfrequenof" the crystal' has been adjusted. Suitable terminals 24extend out through thewally of; the evacuated' chamber' 2t. If thevacuum chamber 213' is of; metal, it also. should have a: contactforconnection to one or the other of. theelectrodes, or to; ground,I asdesired'.

What" is claimed is:

l. A piezoelectric crystal` and holder assembly cnmprisingg.. a;piezoelectric element of the. type adaptedpto respond to frequencywhich; is a funcitimr of; its lengthz-widthf dimensions, .metallici elecThis mounting means may also betrodes disposed in spaced relation onopposite sides of said crystal, a pair of supports constituted ofinsulating material to which said electrodes are respectively secured,and spacing members interposed between said supports and said crystal,saldi electrode supports and said spacing members being constitutedofthe same crystalline` material as said piezoelectric crystal element andhaving substantially the same temperature coecient of expansion as saidelement.

2. A crystal holder comprising a piezoelectric crystal, electrodes forsaid crystal, an insulating spacingmember ofr crystal material which isthe same-as saidpiezoelectric crystal and cut at such an angle'that ithas the same temperature coefficient as the crystal, and clamping meanswhich has anv area small inproportion to the area of said crystal.

3. A piezoelectric crystal holder comprising a. piezoelectric crystal,metallic coated insulated electrodes each' having depressedv surfacesfacing the maior faces of. said crystal, andV insulatn'gg spacing;members of piezoelectric crystal materiali retained in the depressedsurfaces of said crystal..

4'. A crystal holderv of the type` wherein: the crystal oscillates inthe length-widthrmcde, coniY 'prising a rectangular piezoelectriccrystal;` xtec:-n

tangul'ar electrodes for said crystaLan insulating-g spacing member ofpiezoelectric crystalf materia-Ii interposed between said electrodesandlsaidcryss'J tal; an insulating bead held between said spacingzmembers and. said crystal, and' clamping, means` for retaining saidApiezoelectric crystal,. spacing, members and electrodes in operativeposition.

5i A piezoelectricl crystal holder comprising` a piezoelectric crystal,metallic coatedV insulated electrodes each having depressedsurfacescfacing: themaior faces of said crystal, insulatingspacing;members of piezoelectric crystal material refY tained in the depressed'surfaces of said crystal; and' an insulating member having a slottedpor.tion therein and clamping means in said insulat-l ing memberfor`retaining said crystal and: elec,-A trodesin operative-position.

6. A piezoelectric crystal holder comprising; a'7 piezoelectric crystal,metallic coated insulated; electrodes each having depressed surfacesfacing, the major faces'of saidcrystal, insulating spacing members ofpiezoelectric crystal material retained in the depressed surfaces ofsaid crystal, an inf sula'ting crystal holder member having a slottedportion thereinV and clamping means in said; i1rsulating member forretaining said crystal andA electrodes in operative position, andterminal` means passing` through portions of' said insulatzingmember'for making electrical connection to.v the'electrodes;

7. A crystal holder according tov claim 6, wherein the crystal holdermember is enclosed` withina vacuum chamber.

RALPH E; FRANKLIN. WILLIAM A. MILLER.

REFERENCES. @ITER The following references are ci record.' in: the'- leof this patent:

UNITED STATES PATENTS 2,312,746-` Bokovoyy et al.'1 Mar.. 2,1343

